Two types of pistons are used in reciprocating piston air compressors. In a first piston design, the piston has a cylindrical shape and is confined to slide in a cylinder without rotating relative to the cylinder. A connecting rod has an end connected to the piston and has a free end connected to a rotating eccentric. Since the piston cannot rotate with the connecting rod, it is necessary to provide a hinge connection between the connecting rod and the piston through the use of a wrist pin. In order for the compressor to operate, oil lubrication must be provided for the reciprocating piston. The oil also may be sprayed at the bottom of the piston and in chambers inside the piston for cooling. Although the piston may have internal chambers in which cooling oil is sprayed or circulated and to reduce the weight of the piston, there is essentially no induced air flow through the piston because the linear reciprocation of the piston establishes a uniform air pressure across the bottom of the piston.
A second common piston design does not require oil lubrication. The piston is rigidly secured to the connecting rod. The piston is provided with a sufficiently thin profile to allow the piston to wobble or rock in the cylinder with the connecting rod as the piston is reciprocated. A resilient seal is provided around the periphery of the piston to allow the piston to tilt in the cylinder without loss of a gas tight seal between the piston and the cylinder. The sliding seal and a smooth coating on the cylinder reduce friction so that oil lubrication is not required.
The service life of a wobble piston compressor is often limited by the life of the piston seal. Many factors have been found to affect the seal life. In general, seal life is improved both by reducing friction between the seal and the cylinder band by reducing the temperature of the seal. Since heat is released when air is compressed, it has been found important to cool the cylinder and the piston as much as possible to enhance seal life.
The operating efficiency of a compressor also can be degraded by heat. For strength while minimizing weight, the prior rt piston typically has been formed as a relatively thick aluminum casting. Heat absorbed by the piston during the compression stroke is transferred from the piston top to air drawn into the cylinder during the subsequent intake stroke. This causes the air to expand and consequently reduces the volumetric efficiency of the compressor.